Method for testing group III-nitride wafers and group III-nitride wafers with test data

ABSTRACT

The present invention discloses a new testing method of group III-nitride wafers. By utilizing the ammonothermal method, GaN or other Group III-nitride wafers can be obtained by slicing the bulk GaN ingots. Since these wafers originate from the same ingot, these wafers have similar properties/qualities. Therefore, properties of wafers sliced from an ingot can be estimated from measurement data obtained from selected number of wafers sliced from the same ingot or an ingot before slicing. These estimated properties can be used for product certificate of untested wafers. This scheme can reduce a significant amount of time, labor and cost related to quality control.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.12/456,181, filed Jun. 12, 2009, and entitled METHOD FOR TESTING GROUPIII-NITRIDE WAFERS AND GROUP III-NITRIDE WAFERS WITH TEST DATA, whichclaims the benefit of priority to U.S. Provisional Application No.61/131,917, filed Jun. 12, 2008 and entitled METHOD FOR TESTING GROUPIII-NITRIDE WAFERS AND GROUP III-NITRIDE WAFERS WITH TEST DATA,inventors Tadao Hashimoto, Masanori Ikari, and Edward Letts, thecontents of each of which are incorporated by reference herein as if putforth in full below. This application is also related to the PCTApplication No. PCT/US09/03557 filed Jun. 12, 2009 (WO 2009/151642),entitled METHOD FOR TESTING GROUP III-NITRIDE WAFERS AND GROUPIII-NITRIDE WAFERS WITH TEST DATA, inventors Tadao Hashimoto, MasanoriIkari, and Edward Letts, the contents of which are incorporated byreference in their entirety as if put forth in full below.

This application is also related to the following co-pending U.S. patentapplications:

PCT Utility Patent Application Serial No. US2005/024239, filed on Jul.8, 2005, by Kenji Fujito, Tadao Hashimoto and Shuji Nakamura, entitled“METHOD FOR GROWING GROUP III-NITRIDE CRYSTALS IN SUPERCRITICAL AMMONIAUSING AN AUTOCLAVE,”;

U.S. Utility patent application Ser. No. 11/784,339, filed on Apr. 6,2007, by Tadao Hashimoto, Makoto Saito, and Shuji Nakamura, entitled“METHOD FOR GROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS INSUPERCRITICAL AMMONIA AND LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,”,which application claims the benefit under 35 U.S.C. Section 119(e) ofU.S. Provisional Patent Application Ser. No. 60/790,310, filed on Apr.7, 2006, by Tadao Hashimoto, Makoto Saito, and Shuji Nakamura, entitled“A METHOD FOR GROWING LARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS IN ANDLARGE SURFACE AREA GALLIUM NITRIDE CRYSTALS,”;

U.S. Utility Patent Application Ser. No. 60/973,602, filed on Sep. 19,2007, by Tadao Hashimoto and Shuji Nakamura, entitled “GALLIUM NITRIDEBULK CRYSTALS AND THEIR GROWTH METHOD,”;

U.S. Utility patent application Ser. No. 11/977,661, filed on Oct. 25,2007, by Tadao Hashimoto, entitled “METHOD FOR GROWING GROUP III-NITRIDECRYSTALS IN A MIXTURE OF SUPERCRITICAL AMMONIA AND NITROGEN, AND GROUPIII-NITRIDE CRYSTALS GROWN THEREBY,”.

U.S. Utility Patent Application Ser. No. 61/067,117, filed on Feb. 25,2008, by Tadao Hashimoto, Edward Letts, Masanori Ikari, entitled “METHODFOR PRODUCING GROUP III-NITRIDE WAFERS AND GROUP III-NITRIDE WAFERS,”.

U.S. Utility Patent Application Ser. No. 61/058,900, filed on Jun. 4,2008, by Edward Letts, Tadao Hashimoto, Masanori Ikari, entitled “METHODFOR PRODUCING IMPROVED CRYSTALLINITY GROUP III-NITRIDE CRYSTALS FROMINITIAL GROUP III-NITRIDE SEED BY AMMONOTHERMAL GROWTH,”.

U.S. Utility Patent Application Ser. No. 61/058,910, filed on Jun. 4,2008, by Tadao Hashimoto, Edward Letts, Masanori Ikari, entitled“HIGH-PRESSURE VESSEL FOR GROWING GROUP III NITRIDE CRYSTALS AND METHODOF GROWING GROUP III NITRIDE CRYSTALS USING HIGH-PRESSURE VESSEL ANDGROUP III NITRIDE CRYSTAL,”.

which applications are incorporated by reference herein.

BACKGROUND

1. Field of the Invention

The invention is related to the production method of group III-nitridewafers using ammonothermal method and subsequent testing method and thegroup III-nitride wafers with test certificate data.

2. Description of the Existing Technology

(Note: This patent application refers several publications and patentsas indicated with numbers within brackets, e.g., [x]. A list of thesepublications and patents can be found in the section entitled“References.”)

Gallium nitride (GaN) and its related group III alloys are importantmaterials for various opto-electronic and electronic devices such aslight emitting diodes (LEDs), laser diodes (LDs), microwave powertransistors, and solar-blind photo detectors. Currently LEDs are widelyused in cell phones, indicators, displays, and LDs are used in datastorage discs. However, the majority of these devices are grownepitaxially on heterogeneous substrates, such as sapphire and siliconcarbide since GaN wafers are extremely expensive compared to theseheteroepitaxial substrates. The heteroepitaxial growth of groupIII-nitride causes highly defected or even cracked films, which hindersthe realization of high-end optical and electronic devices, such ashigh-brightness LEDs for general lighting or high-power microwavetransistors.

To solve problems caused by heteroepitaxy, it is helpful to utilizesingle crystalline group III-nitride wafers sliced from bulk groupIII-nitride crystal ingots. For a majority of devices, singlecrystalline GaN wafers are favored because it is relatively easy tocontrol the conductivity of the wafer, and a GaN wafer will providesmall lattice/thermal mismatch with device layers. However, due to thehigh melting point and high nitrogen vapor pressure at high temperature,it has been difficult to grow group III-nitride crystal ingots. Growthmethods using molten Ga, such as high-pressure high-temperaturesynthesis [1,2] and sodium flux [3,4], have been proposed to grow GaNcrystals. However, the crystal shape grown in molten Ga becomes a thinplatelet because molten Ga has low solubility of nitrogen and a lowdiffusion coefficient of nitrogen.

Currently, only hydride vapor phase epitaxy (HVPE) can producecommercial GaN wafers. In HVPE, a thick (0.5-3 mm) GaN film is grown ona heteroexpitaxial substrate such as sapphire or gallium arsenide. Thesubstrate is subsequently removed and a free-standing GaN wafer isfinished by grinding and polishing. Since the GaN wafer is produced oneby one on each heterogeneous substrate, each produced GaN wafer istested before shipping. However, as the production volume increases, itbecomes extremely time consuming to test all wafers before shipping.

SUMMARY OF THE INVENTION

The present invention discloses a new production and testing method forgroup III-nitride wafers. An ammonothermal method, which is a solutiongrowth method using high-pressure ammonia as a solvent, has demonstratedsuccessful growth of real bulk GaN ingots [see generally references 5-8for ammonothermal growth]. By utilizing the ammonothermal method, GaNwafers can be obtained by slicing the bulk GaN ingots. Since thesewafers originate from the same ingot, these wafers have similarproperties/qualities. Therefore, test data obtained from the ingotbefore slicing or one or more selected wafers from an ingot canrepresent properties of other wafers from the same ingot. This schemewill cut significantly the amount of time, labor and cost related toquality control.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a flow chart for the production of group III-nitride waferswith improved test scheme in which selected wafers are tested from oneingot;

FIG. 2 is a flow chart for the production of group III-nitride waferswith improved test scheme in which an ingot is tested before slicing.

FIG. 3 is a photograph of GaN ingot grown by the ammonothermal method.

FIG. 4 is a photograph of GaN wafers sliced from the GaN ingot in FIG.3,

FIG. 5 is full width half maximum of x-ray rocking curve measurementdepending on the measurement position. For the bulk ingot, position 1Gis on the Ga-face of the ingot and position 4N is on the N-face of theingot. For the sliced wafers position 1G is on the Ga-face of the wafer1 (see FIG. 4), position 1N is on the N-face of the wafer 1, position 20is on the Ga-face of the wafer 2, position 2N is on the N-face of thewafer 2, position 3G is on the Ga-face of the wafer 3, position 3N is onthe N-face of the wafer 3, position 4G is on the Ga face of the wafer 4,position 4N is on the N-face of the wafer 4. The seed crystal waslocated between position 2N and 3G as indicated in the figure.

FIG. 6 is a schematic drawing of the high-pressure vessel in thisinvention. In the figure each number represents the followings:

-   -   1. High-pressure vessel    -   2. Lid    -   3. Clamp    -   4. Gasket    -   5. Heater(s) for the crystallization region    -   6. Heater(s) for the dissolution region    -   7. Flow-restricting baffle(s)    -   8. Nutrient basket    -   9. Nutrient    -   10. Seed crystals    -   11. Flow-restricting baffle(s)    -   12. Blast containment enclosure    -   13. Valve    -   14. Exhaust tube    -   15. Device to operate valve

FIG. 7 is a photograph comparing colorations depending on mineralizersand additives.

FIG. 8 is a photograph of GaN crystal with Mg additive.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

1. Technical Description of the Invention

GaN wafers are used for blue/green/white LEDs and blue LDs, whoseperformance and reliability greatly depend on the wafer quality. Thus,customers usually require some sort of quality certificate on products.Currently, a commercial GaN wafer is produced by growing a thick GaNfilm on heteroepitaxial substrate such as sapphire or gallium arsenideby hydride vapor phase epitaxy (HVPE) and subsequently removing thesubstrate. The characteristics of each wafer suffer from run-to-runfluctuation of HYPE growth condition. Therefore, to ensure compliance toquality specifications for GaN wafers, it has been necessary to test allHVPE-grown wafers after fabrication. However, as production volumeincreases, testing all wafers takes too much time, labor and cost. Torealize low-cost, high-quality GaN wafers, it is necessary to develop agrowth method to obtain GaN ingots so that GaN wafers sliced from theingot show similar characteristics. Recently, it was proven that theammonothermal method has sufficient feasibility to grow GaN ingots.Through our recent development of GaN ingot by the ammonothermal method,we found that characteristics of wafers sliced from one ingot showproperties with anticipated certain trend. Therefore, we haveinvestigated the possibilities of testing an ingot or selected number ofwafers from an ingot to represent other wafers in the same ingot.

Gallium nitride has a wurtzite crystal structure with crystal facespolar and non-polar to the Group III element or elements and thenitrogen incorporated into the crystal structure. Samples or substratessuch as wafers may be out from an ingot along a crystal face, or samplesmay be cut at a small angle to a crystal face, such as about 3 degrees.Generally, the angle is less than about 10 degrees, and the angle may bein the range of about 5 to 10 degrees.

An ingot is typically a sufficiently large single crystal material fromwhich at least 4 wafers may be cut. Of course, more wafers may be cutfrom an ingot, such as 5, 6, 7, 8, 9, 10, or more wafers may be cut froman ingot of Group III-nitride material.

Equipment and methods that can be used to grow ingots from which a largenumber of wafers may be cut are disclosed in copending U.S. and PCTapplication numbers [not yet assigned] both filed on Jun. 4, 2009 andboth entitled “High-Pressure Vessel For Growing Group III NitrideCrystals And Method Of Growing Group III Nitride Crystals UsingHigh-Pressure Vessel And Group III Nitride Crystal”, inventors TadaoHashimoto, Edward Letts, and Masanori Ikari, the contents of which areincorporated by reference herein in their entirety as if put forth infull below. The equipment and methods are also summarized below.

In one instance, a high-pressure vessel having an inner diameter of 1inch was used to grow bulk GaN ingots. All necessary sources andinternal components including polycrystalline GaN nutrient held in a Nibasket, 0.4 mm-thick single crystalline GaN seeds, and 3 to 6flow-restricting baffles were loaded into a glove box together with thehigh-pressure vessel. The glove box is filled with nitrogen and theoxygen and moisture concentration is maintained to be less than 1 ppm.Since the mineralizers are reactive with oxygen and moisture, themineralizers are stored in the glove box at all times. NaNH₂ or Na wasused as a mineralizer. After loading the mineralizer into thehigh-pressure vessel, three baffles together with seeds and nutrientwere loaded. After sealing the high-pressure vessel, it was taken out ofthe glove box. Then, the high-pressure vessel was connected to agas/vacuum system, which can pump down the vessel as well as supply NH₃to the vessel. The high-pressure vessel was heated at 126° C. andevacuated with a turbo molecular pump to achieve pressure less than1×10⁻⁵ mbar. The actual pressure before filling ammonia wasapproximately 1.6×10⁻⁶ mbar. In this way, residual oxygen and moistureon the inner wall of the high-pressure vessel were removed. After this,the high-pressure vessel was chilled with liquid nitrogen and NH₃ wascondensed in the high-pressure vessel. Approximately 38.5 g of NH₃ wascharged in the high-pressure vessel. After closing the high-pressurevalve of the high-pressure vessel, it was transferred to a two zonefurnace. The high-pressure vessel was heated up. The crystallizationregion was maintained at 575-595° C. and the nutrient region wasmaintained at 510° C. The resulting pressure was approximately 22,000psi (150 MPa). After designated growth time, ammonia was released andthe high-pressure vessel was opened.

FIG. 3 shows a GaN ingot grown by the ammonothermal method and FIG. 4shows the wafers sliced along c plane. There was a slight misorientation(up to +1-5 degree) for the cut. The grown bulk crystal ingot was testedwith x-ray diffractometer on the surface of Ga-polar c plane (Ga-face)and N-polar c plane (N-face). A typical index used for quality controlis a full width half maximum (FWHM) of (002) diffraction peak in rockingcurve measurement. The FWHM values measured from the bulk GaN crystalwas 6238, and 927 arcsecond on the Ga-face and the N-face, respectively.The FWHM values measured from the original seed crystal was 420 and 381arcsecond on the Ga-face and the N-face, respectively.

After slicing the bulk crystal along c plane with a wire saw, the each cplane wafer was tested with x-ray diffractometer. The measurement datais plotted in FIG. 5 together with the data of the original seed crystaland the grown bulk crystal. As clearly seen in the figure, the propertyof each wafer is within the range of values obtained from the ingotmeasurement. Therefore, is it practical to use the range of themeasurement values obtained from the ingot measurement for each wafer'sspecification. Furthermore, we can estimate the value by studying acertain trend of the measurement value. For example, in the case ofusing c plane seeds, it has turned out that the x-ray FWHM value becameworse for wafers further from the seed crystal (located betweenpositions 2N and 3G) for a portion grown on the Ga-side of the seed(positions 1G, 1N, 2G, 2N), whereas the value did not change much forwafers sliced from the portion grown on the N-side of the seed (forpositions 3G, 3N, 4G, 4N). Therefore, one can certify that themeasurement value of the wafers on the Ga-side is within the range ofthe values obtained from the Ga-face and the N-face of the ingot, andthe measurement value of the wafers sliced from the N-side is similar tothat obtained from the N-face of the ingot.

As demonstrated in FIG. 5, one can also measure selected wafers toobtain representative measurement values for other wafers. By using thevalue on outer-most wafers (i.e. 1G, 1N, 4G and 4N), one can interpolatethe quality value of other wafers with less error bar.

Because the ammonothermal method utilizes seed crystals, the test dataof the seed crystal before the ammonothermal growth will also representsthe wafer characteristics close to the original seed. Therefore, testdata from a bulk crystal before slicing or test data from outer-mostwafers together with the test data of the original seed and the waferlocation will provide more accurate quality values for untested wafersbetween the seed and the outer-most wafers.

If one bulk ingot produces much more wafers, one can choose wafers formeasurement between the outer-most wafers and the original seedposition. The quality value of untested wafers can be certified within acertain error bar by providing the measured values and the location ofthe wafer relative to the measured wafers.

The analysis and correlation may also utilize an end piece of an ingotthat might not generally be considered a wafer. In some analyses, onlyone end piece is used in deriving a correlation. In other analyses, bothend pieces may be used in deriving a correlation, and sometimesinformation from only the end pieces is used to derive a correlation.

Information on a physical property of the seed material may or may notbe used in correlating data on the physical property and using thecorrelation to provide information on untested wafers.

The physical properties that are tested and correlated according to amethod herein may be any one or more selected from peak full width halfmaximum of x-ray diffraction, peak position and/or intensity ofphotoluminescence, impurity signal from secondary ion mass spectroscopy,conductivity, carrier concentration, and mobility of carriers.

Various statistical correlations may be utilized. For instance, linearor non-linear correlations such as normal or weighted least squares,exponential, and averaging may be utilized to predict values of aproperty for wafers not tested.

A different statistical correlation may be used for wafers cut from aningot on one polar side of a seed material than for wafers cut from theopposite polar side of a seed material used to form the ingot. Forinstance, a linear correlation may be used for a property of waferstaken from the N-polar side of the ingot, and a different linear, aleast squares, or an exponential correlation may be used for waferstaken from the Group III-polar side of the wafer. In examining the datafrom FIG. 5, a linear correlation may adequately describe the data forwafers taken from the N-polar side of the seed material used to grow theingot, whereas a different linear or a least-squares fit may be used toestimate properties for untested wafers taken from the ingot taken fromthe Ga-polar side of the seed material.

A property of the wafers, seed, and/or ingot may be assessed usingX-rays. One such method is X-ray diffraction, and especially rockingcurve X-ray analysis. Other test methods include the use of laser light,electron beam, photo luminescence, cathode luminescence, secondary ionmass spectroscopy, Hall measurement, capacitance-voltage measurement,and other tests used to characterize Group III-nitride materials.

Wafers may be packaged with representative test data taken from aselected number of wafers or the ingot itself. Test data for the seedmay instead be supplied with packaged wafers.

2. Advantages and Improvements

The present invention disclosed a new testing method of groupIII-nitride wafers with improved throughput. In the conventionalproduction method of GaN wafers by HVPE, each produced wafers must betested because wafers are produced one by one. In the current inventionutilizing the ammonothermal method, the property of wafers sliced fromone ingot can be easily estimated by measuring the ingot or selectednumber of wafers. This selected tested scheme reduces significant amountof time, labor and cost.

3. Equipment and Methods

A high-pressure vessel suitable for use in ammonothermal growth ofingots of a Group III-nitride material that produce a large number ofwafers may be made from a raw material such as superalloy rod or billet.Conventional vessels are limited in size today when this type of rawmaterial is used to make the high-pressure vessel. Described below is avessel that allows ingots to be formed that may produce a number ofwafers as discussed above.

The vessel may have one or more clamps to seal the vessel. A clamp maybe formed of a metal or alloy having grain flow in a radial direction inthe clamp. This configuration enables the vessel to be much greater insize than vessels today.

The high-pressure vessel may be divided into at least three regions byflow-restricting devices such as baffles. In this embodiment, the vesselhas a nutrient region in which a nutrient such as polycrystalline GaN orother feed material is dissolved or supplied. The vessel also has acrystallization region in which the group III-nitride material iscrystallized upon a seed material. The vessel also has a buffer regionbetween the nutrient region and the crystallization region, one or morecool zones adjacent to the crystallization zone and/or the nutrient zoneand in the vicinity of a clamp, or both.

Some procedures below can be used to grow crystals with improved purity,transparency and structural quality. Alkali metal-containingmineralizers are charged with minimum exposure to oxygen and moistureuntil the high-pressure vessel is filled with ammonia. Several methodsto reduce oxygen contamination during the process steps are presented.Also, back etching of seed crystals and a temperature ramping scheme toimprove structural quality are discussed.

In addition, a method below reduces parasitic deposition ofpolycrystalline GaN on the inner surface of the pressure vessel whileoptionally improving structural quality and without reducing growthrate. Because of the retrograde solubility of GaN in the ammonothermalgrowth using basic mineralizers, the temperature in the crystallizationregion is conventionally set higher than the temperature in the nutrientregion. We, however, discovered that GaN can be grown by setting thetemperature of the crystallization region slightly lower than thetemperature of the nutrient region. By utilizing this “reverse”temperature setting for growth, parasitic deposition of GaN on thereactor wall was greatly reduced. Moreover, the structural quality ofgrown crystal was improved without sacrificing growth rate.

The present invention provides a design to achieve a high-pressurevessel suitable for mass production of group III-nitride crystals by theammonothermal method. With the limited size of available Ni—Cr basedprecipitation hardenable superalloy, innovative designs are presented tomaximize the vessel size. Also, methods of growing improved-qualitygroup III nitride crystals are presented.

In one instance, the reactor has: a body defining a chamber; and a firstclamp for sealing an end of the reactor, wherein the first clamp isformed of a metal or alloy having a grain flow in a radial direction inthe clamp.

In another instance, the reactor for ammonothermal growth of a groupIII-nitride crystalline ingot has: a body defining a chamber; a firstheater for heating a nutrient region of the reactor; a second heater forheating a crystallization region of the reactor; a third region selectedfrom a buffer region and an end region; and at least one baffleseparating the crystallization region from the nutrient region or thecrystallization region from an end region of the reactor, with thequalifications that the buffer region when present is defined by aplurality of baffles comprising a first end baffle and an opposite endbaffle with one or more optional baffles therebetween, and a distancefrom the first end baffle to the opposite end baffle of the plurality isat least ⅕ of an inner diameter of the reactor; and the reactor furthercomprises a first clamp at the end region when the end region ispresent.

Any of the reactors described herein may have a single opening at anend. Alternatively, any of the reactors described herein may have two ormore openings, with one opening at each end of the reactor body. Thereactor may be generally cylindrical, or the reactor may be formed inanother shape such as spherical or semispherical.

A reactor as discussed herein may have a buffer region and/or one ormore cooled end regions.

A clamp for the reactor, such as a clamp positioned in an end region,may be formed of a metal or alloy having a grain flow in a radialdirection in the clamp. The clamp may be formed of a superalloy such asa Ni—Cr superalloy. Alternatively, a clamp may be formed of highstrength steel.

A reactor may have a first heater that is configured to maintain thenutrient region of the reactor at a temperature near but greater than atemperature of the reactor in a crystallization region of the reactorwhen the reactor is configured to grow a gallium nitride crystal havingretrograde solubility in supercritical ammonia.

The reactor may be configured so that the first heater maintains thenutrient region no more than about 20° C. greater than the temperatureof the crystallization region.

Any reactor may have a second clamp for sealing a second end of thereactor. The second clamp may be identical to the first clamp discussedabove.

The clamp may be formed in two or three or more pieces so that the clampcan be disassembled and removed from the reactor. The pieces may be heldtogether by bolts formed of the same material as the clamps.

A reactor for ammonothermal growth of a group III-nitride crystallineingot may also have a body defining a chamber; and a mineralizer in anencapsulant, wherein the encapsulant is an oxygen- and water-impermeablematerial capable of rupturing under growth conditions in the reactor.

As discussed in further detail below, the encapsulant may be a metal oralloy coating on the mineralizer that softens or melts at crystal growthconditions in the reactor.

The encapsulant may instead or additionally comprise a water- andoxygen-impermeable container that ruptures under crystal growthconditions in the reactor.

The container may have a chamber which contains the encapsulant and hasa pressure much less than a pressure within the reactor under crystalgrowth conditions, so that the pressure on the container exceeds itsyield strength when the reactor attains operating pressure andtemperature.

Any of the reactors discussed herein may operate using an ammonobasicsupercritical solution or an ammonoacidic supercritical solution. Themineralizer may therefore be basic or acidic. Such mineralizers arewell-known in the art.

Also discussed herein is a method of forming a group III-nitridecrystalline material. The method includes:

providing a mineralizer substantially free of oxygen and water to areaction chamber of an ammonothermal growth reactor

evacuating the chamber

growing the group III-nitride crystalline material in the chamber.

Any method herein may include providing an oxygen getter in the reactionchamber.

A method of forming gallium nitride crystalline material may comprise

heating a nutrient comprising polycrystalline GaN in a nutrient regionof a reactor

heating a seed material in a crystallization region of the reactor

dissolving the polycrystalline GaN in supercritical ammonia

depositing the dissolved GaN on the seed material to grow the galliumnitride crystalline material

wherein the temperature of the nutrient region is greater than but nearthe temperature of the crystallization region.

In such a method, the difference in temperature between the nutrientregion and the crystallization region may be no more than about 50, 40,30, 25, 20, 15, or 10° C. during the act of depositing the dissolved GaNon the seed material.

Any method discussed herein may further comprise back-etching thenutrient prior to the act of growing the crystalline material.

Reactors and methods are discussed in greater detail below.

Technical Description

Since there is a size limitation in forged materials, it is not possibletoday to obtain large diameter rod of various metals or alloys such asprecipitation hardenable Ni—Cr based superalloy from which a pressurevessel suitable for ammonothermal growth of a group III-nitridecrystalline material can be made. For example, the maximum diameter ofas-cast R-41 billet is 17 inches, and this billet is forged (e.g.round-forged, where the forging pressure is applied along the radialdirection) into 12-inch diameter rod before making the reactor. This isthe maximum diameter of forged rod in state-of-the-art technology andmeans that the maximum outer diameter of the high-pressure vessel can beclose to 12 inches. However, this maximum diameter cannot be realizedusing existing methods because it is also necessary to form clamps forthe high-pressure vessel from the forged metal or alloy.

For a high-pressure vessel of this scale, a clamp-type closure is saferand more practical due to the increased cover load (i.e. the forceapplied to lids at the conditions present in ammonothermal crystalgrowth). A screw-type closure is not practical for a high-pressurevessel having an inner diameter larger than 2 inches because the screwcap requires too many threads to hold the cover load.

However, since the clamp diameter must be larger than the outer diameterof the high-pressure vessel, the maximum outer diameter of thehigh-pressure vessel is limited by the clamp diameter. Using R-41 forexample, the clamp diameter of 12 inch results in a high-pressure vesselwith 4-6 inch outer diameter and 2-3 inch inner diameter. Although onecan produce 2-inch wafers of GaN with such high-pressure vessel, theproductivity is not large enough to compete with other growth methodssuch as hydride vapor phase epitaxy (HVPE).

Example 1

To solve the above-mentioned problem, a few methods to obtain largerclamps were sought and we discovered it is possible to manufacture alarge diameter disk with e.g. precipitation hardenable Ni—Cr basedsuperalloy or other metal or alloy by forging as-cast billet alonglongitudinal direction. With this method, a 17-inch as-cast billet isforged along the longitudinal direction to form a 20-inch diameter,1-foot thick disk. The forging process usually creates a directionalmicrostructure (grain flow) which determines tensile strength of thematerial.

The grain flow of the round rod to construct the body and lids istypically along the longitudinal direction whereas the grain flow of thedisks to construct the clamps is along the radial direction. While onemight expect a disk with radial grain flow rather than axial grain flowto not have sufficient tensile strength to reliably clamp a lid to areactor body during use, this difference does not have a significantimpact on the reliability of the high-pressure vessel as long as thehigh-pressure vessel is designed with sufficient safety factor. Withthis method, we designed a high-pressure vessel with 11.5″ outerdiameter, 4.75″ inner diameter, and height larger than 47.5″. Since theheight of the high-pressure vessel is large, it is necessary to bore ahole from both ends to form a chamber within the reactor vessel.Therefore a high-pressure vessel might require two lids, one on eachend. The clamp diameter in this instance is 20″ and the thickness is12″, which ensures sufficient strength to hold a lid during crystalgrowth. A large clamp such as this may be formed of two or more clampsections that are e.g. bolted together to form the finished clamp.

A large reactor such as the one discussed above provides greateropportunities to improve the quality and size of larger-diameter crystalgrown in the reactor. Hot ammonia circulates inside the high-pressurevessel and transfers heat. To establish preferable temperaturedistribution, flow-restricting baffles may be used. Unlike theconventional ammonothermal reactor, the invented high-pressure vesselmay be equipped with a flow restricting baffle to cool the bottom region(11 in FIG. 6). In this way, the temperature around the seal and theclamp is maintained lower than that of the crystallization region toimprove reliability. In addition, this temperature distribution preventsunnecessary precipitation of GaN on the bottom lid.

The large reactor design also allows a large buffer region to beincorporated between the crystallization region and the nutrient region.This buffer region may comprise multiple baffles where holes or openingsin a baffle are offset from holes or openings in adjacent baffles.

These baffles increase the average residence time within the bufferregion while providing some regions of relatively stagnant flow andother regions of vortex flow for ammonia passing through the regionsbetween baffles. The average flow-rate from the nutrient region to thecrystallization region in the larger-diameter reactor may also be lessthan the average flow-rate from the nutrient region to thecrystallization region in a smaller-diameter reactor.

The buffer region may also be positioned primarily or exclusivelyadjacent to the heaters for the nutrient region for a reactor configuredto provide a lower temperature in the nutrient region and highertemperature in the crystallization region to take advantage of theretrograde solubility of e.g. GaN in supercritical ammonia. Or, thebuffer region may extend past heaters for the crystallization zone toprovide earlier heating or cooling of the supercritical ammonia, so thatthe ammonia solution is more likely to be at the desired temperaturewhen the solution encounters the seed material.

The slower ammonia flow supplied in the crystallization region with itsincreased residence time in the buffer zone, of the supersaturatedammonia, may thus increase the growth rate as shown in Example 3.

As the size of the high-pressure vessel increases, the total internalvolume becomes large. Thus, it is important to mitigate possible blasthazards. The equivalent blast energy in this example is estimated to beas high as 9 lbs TNT. In this example, we designed a blast containmentenclosure having ½ inch-thick steel wall (12 in FIG. 6). One importantthing is that the ammonia in the high-pressure vessel can be released byremote operation of the high-pressure valve (13 in FIG. 6). The valvecan be remote-operated via either mechanical, electrical, or pneumaticway.

Example 2

Another method to realize a large diameter high-pressure vessel is touse high-strength steel for the clamp material. Although the maximumpractical temperature for high-strength steel (such as 4140) is 550° C.,the maximum available size is large enough to fabricate clamps forhigh-pressure vessel having 12-inch outer diameter. In this case extracaution is necessary to control the clamp temperature. An end region inthe reactor that has one or more baffles to impede ammonia flow permitsammonia to cool in an end region, reducing the reactor temperature inthat region and allowing a clamp to be formed of a material such as highstrength steel that might not otherwise be used to form a clamp. Also,appropriate anti-corrosion coating is advisable in case of ammonia leaksince steel is susceptible to corrosion by basic ammonia.

Example 3

In this example, a high-pressure vessel having an inner diameter of 1inch was used to demonstrate the enhanced growth rate by having one ormore buffer regions between the crystallization region and the nutrientregion. Unlike the Examples 1, 2, this high-pressure vessel has an openend on one side only. The chamber of the high-pressure vessel is dividedinto several regions. From bottom to top, there were a crystallizationregion, buffer regions, and a nutrient region. GaN crystals were grownwith 2 buffer regions, 5 buffer regions, and 8 buffer regions and thegrowth rate in each condition was compared. First, the high-pressurevessel was loaded with 4 g of NaNH₂, a seed crystal suspended from aflow-restricting baffle having 55 mm-long legs (the first baffle). Tocreate buffer regions, 2 flow restricting baffles with 18 mm legs, 5flow restricting baffles with 18 mm legs or 8 flow restricting baffleswith 10 mm legs were set on top of the first baffle (i.e. buffer regionswith height of 18 mm were created for the first two conditions andbuffer regions with height of 10 mm were created for the lastcondition). All of the baffles had ¼″ hole at the center and the totalopening was about 14%. The baffle holes in this instance were not offsetfrom one another. On top of these baffles, a Ni basket containing 10 gof polycrystalline GaN was placed and the high-pressure vessel wassealed with a lid. All of these loading steps were carried out in aglove box filled with nitrogen. The oxygen and moisture concentration inthe glove box was maintained to be less than 1 ppm. Then, thehigh-pressure vessel was connected to a gas/vacuum system, which canpump down the vessel as well as supply NH₃ to the vessel. First, thehigh-pressure vessel was pumped down with a turbo molecular pump toachieve pressure less than 1×10⁻⁵ mbar. The actual pressure in thisexample was 2.0×10⁻⁶, 2.4×10⁻⁶ and 1.4×10⁶ mbar for conditions with 2,5, and 8 buffer regions, respectively. In this way, residual oxygen andmoisture on the inner wall of the high-pressure vessel were removed.After this, the high-pressure vessel was chilled with liquid nitrogenand NH₃ was condensed in the high-pressure vessel. About 40 g of NH₃ wascharged in the high-pressure vessel. After closing the high-pressurevalve of the high-pressure vessel, the vessel was transferred to a twozone furnace. The high-pressure vessel was heated to 575° C. of thecrystallization zone and 510° C. for the nutrient zone. After 7 days,ammonia was released and the high-pressure vessel was opened. The growthrate for conditions with 2, 5, and 8 buffer regions were 65, 131, and153 microns per day, respectively. The growth rate was increased withincreased number of buffer regions between the crystallization regionand the nutrient region. One reason for the enhanced growth rate is thatthe convective flow of ammonia became slower as the buffer regionsincreased. Another reason is creating larger temperature difference byrestricting heat exchange between the nutrient region andcrystallization region. Therefore, adjusting the opening of the baffle,changing the position of holes on the baffle, adjusting the height ofthe buffer region is expected to have a similar effect on growth rate.To enhance the growth rate effectively, it is desirable to maintain theheight of the buffer region larger than ⅕ of the inner diameter of thehigh-pressure vessel so that enough space is created to promotestagnant/vortex flow of ammonia.

Technical Description

Here several methods to improve purity, transparency, and structuralquality of GaN grown by the ammonothermal method are presented. Thefollowings examples help illustrate further principles of the claimedinvention, as do the preceding examples.

Example 4

In this example, a high-pressure vessel having an inner diameter of 1inch was used to demonstrate the ammonothermal growth steps withhigh-vacuum pumping before ammonia filling. Unlike the Examples 1, 2,this high-pressure vessel has an open end on one side only. Allnecessary sources and internal components including 10 g ofpolycrystalline GaN nutrient held in a Ni basket, 0.3 mm-thick singlecrystalline GaN seeds, and three flow-restricting baffles were loadedinto a glove box together with the high-pressure vessel. The glove boxis filled with nitrogen and the oxygen and moisture concentration ismaintained to be less than 1 ppm. Since the mineralizers are reactivewith oxygen and moisture, the mineralizers are stored in the glove boxat all times. 4 g of NaNH₂ was used as a mineralizer. After loading themineralizer into the high-pressure vessel, three baffles together withseeds and nutrient were loaded. After sealing the high-pressure vessel,it was taken out of the glove box. Then, the high-pressure vessel wasconnected to a gas/vacuum system, which can pump down the vessel as wellas supply NH₃ to the vessel. The high-pressure vessel was heated at 110°C. and evacuated with a turbo molecular pump to achieve pressure lessthan 1×10⁻⁵ mbar. The actual pressure before filling ammonia was2.0×10⁻⁶ mbar. In this way, residual oxygen and moisture on the innerwall of the high-pressure vessel were removed. After this, thehigh-pressure vessel was chilled with liquid nitrogen and NH₃ wascondensed in the high-pressure vessel. Approximately 40 g of NH₃ wascharged in the high-pressure Vessel. After closing the high-pressurevalve of the high-pressure vessel, it was transferred to a two zonefurnace. The high-pressure vessel was heated up. The crystallizationregion was maintained at 575° C. and the nutrient region was maintainedat 510° C. After 7 days, ammonia was released and the high-pressurevessel was opened. The grown crystals showed dark brownish color (thetop sample in FIG. 7) and the oxygen concentration measured withsecondary ion mass spectroscopy (SIMS) was 1.2-2.8×10²⁰ cm⁻³. In thisexample, pumping and heating of the high-pressure vessel did notcontribute to oxygen reduction. This is because the NaNH₂ mineralizercontained significant amount of oxygen/moisture as shown in the nextexample. After minimizing oxygen/moisture content of mineralizers,pumping and heating of the high-pressure vessel is necessary to minimizepossible source of oxygen in the process.

Example 5

In this example, a high-pressure vessel having an inner diameter of 1inch was used to demonstrate the ammonothermal growth steps using Namineralizer with high-vacuum pumping before ammonia filling. Unlike theExamples 1, 2, this high-pressure vessel has an open end on one side.All necessary sources and internal components including 10 g ofpolycrystalline GaN nutrient held in a Ni basket, 0.3 mm-thick singlecrystalline GaN seeds, and six flow-restricting baffles were loaded intoa glove box together with the high-pressure vessel. The glove box isfilled with nitrogen and the oxygen and moisture concentration ismaintained to be less than 1 ppm. Since the mineralizers are reactivewith oxygen and moisture, the mineralizers are stored in the glove boxat all times. 2.3 g of Na cube was used as a mineralizer. The surface ofthe Na was “peeled” to remove oxide layer. However, even in theglovebox, the fresh surface changed color in seconds, which representedinstantaneous oxidation of the Na surface. After loading the mineralizerinto the high-pressure vessel, six baffles together with seeds andnutrient were loaded. After sealing the high-pressure vessel, it wastaken out of the glove box. Then, the high-pressure vessel was connectedto a gas/vacuum system, which can pump down the vessel as well as supplyNH₃ to the vessel. The high-pressure vessel was evacuated at roomtemperature with a turbo molecular pump to achieve pressure less than1×10⁻⁵ mbar. The actual pressure before filling ammonia was 1.5×10⁻⁶mbar. In this way, residual oxygen and moisture on the inner wall of thehigh-pressure vessel were removed. After this, the high-pressure vesselwas chilled with liquid nitrogen and NH₃ was condensed in thehigh-pressure vessel. Approximately 40 g of NH₃ was charged in thehigh-pressure vessel. After closing the high-pressure valve of thehigh-pressure vessel, it was transferred to a two zone furnace. Thehigh-pressure vessel was heated up. The crystallization region wasmaintained at 575° C. and the nutrient region was maintained at 510° C.After 7 days, ammonia was released and the high-pressure vessel wasopened. The coloration of the grown crystals was improved (the secondsample from the top in FIG. 7). The oxygen concentration measured bysecondary ion mass spectroscopy (SIMS) was 0.7-2.0×10¹⁹ cm³, which wasmore than one order of magnitude lower level than the sample in Example4. Using mineralizer with low oxygen/moisture content together withpumping the high-pressure vessel is shown to be important to minimizeoxygen concentration in the crystal.

Example 6

As shown in Example 5, reducing oxygen/moisture content of mineralizersis vital for improving purity of GaN. As explained in Example 5, Nasurface was oxidized quickly even in the glove box. Therefore, it isimportant to reduce exposed surface of Na or alkali metal mineralizers.Oxidation of Na was reduced by melting the Na in Ni crucible. A hotplate was introduced in the glovebox mentioned in the Example 5 to heatNi crucible under oxygen and moisture controlled environment. Afterremoving surface oxide layer of Na, it was placed in a Ni crucibleheated on the hot plate. When the Na melts, its surface fully contactson the bottom and wall of the crucible. Ni foil is placed on the topsurface of the Na to minimize exposed surface of Na. Since the Nicrucible is small enough to fit the autoclave and Ni is stable in thegrowth environment, the Ni crucible containing Na can be loaded in thehigh-pressure vessel directly. In this way, most of surface of Na iscovered with Ni, therefore oxidation of Na is minimized. Any metal whichis stable in the supercritical ammonia can be used for cruciblematerial. Typical metal includes Ni, V, Mo, Ni—Cr superalloy, etc.

Example 7

Another method to cover fresh surfaces of alkali metal is to punchthrough alkali metal cake with hollow metal tubing. This way, thesidewall of the alkali metal is automatically covered with the innerwall of the tubing, thus reduces surface area exposed to theenvironment. For example, a slab-shaped Na sandwiched with Ni foil canbe prepared by attaching Ni foil immediately after cutting top or bottomsurface of the slab. Although the sidewall of the slab is not covered,punching through the Na slab with Ni tubing can create Na cake withsidewall attaching to Ni and top and bottom surface covered with Nifoil. In this way, oxygen/moisture content of alkali metal mineralizercan be greatly reduced.

Example 8

When the high-pressure vessel becomes large, it is impossible to loadmineralizers to the high-pressure vessel in a glove box. Therefore it isnecessary to take alternative procedure to avoid exposure ofmineralizers to oxygen/moisture. One method is to use an airtightcontainer which cracks under high pressure. Here is one example of theprocedure. Mineralizer is loaded into the airtight container in theglove box. If alkali metal is used, it can be melted and solidified inthe way explained in Example 6. Then, the high pressure vessel is loadedwith the airtight container and all other necessary parts and materialsunder atmosphere. After sealing the high-pressure vessel, it is pumpeddown and heated to evacuate residual oxygen/moisture. Ammonia is chargedin the high-pressure vessel and the high-pressure vessel is heated togrow crystals. When internal ammonia pressure exceeds cracking pressureof the airtight container, the mineralizer is released into the ammonia.In this way, mineralizers can be added to ammonia without exposing themto oxygen and moisture.

Example 9

To further reduce oxygen concentration, it is effective to remove oxygenin the high-pressure vessel after the high-pressure vessel is sealed andbefore the ammonia is charged. One practical procedure is to pump andbackfill reducing gas in the high-pressure vessel. The reducing gas suchas ammonia and hydrogen reacts with residual oxygen in the high-pressurevessel and form water vapor. Therefore, it is further effective to heatthe high-pressure vessel to enhance the reduction process.

Example 10

In this example, a high-pressure vessel having an inner diameter of 1inch was used to demonstrate the ammonothermal growth steps with Ceaddition as an oxygen getter. Unlike the Examples 1, 2, thishigh-pressure vessel has an open end on one side. All necessary sourcesand internal components including 5 g of polycrystalline GaN nutrientheld in a Ni basket, 0.4 mm-thick single crystalline GaN seeds, and sixflow-restricting baffles were loaded into a glove box together with thehigh-pressure vessel. The glove box is filled with nitrogen and theoxygen and moisture concentration is maintained to be less than 1 ppm.Since the mineralizers are reactive with oxygen and moisture, themineralizers are stored in the glove box at all times. 2.4 g of Na wasused as a mineralizer. After loading the mineralizer into thehigh-pressure vessel, six baffles together with seeds and nutrient wereloaded. Then, 0.4 g of Ce powder was added. After sealing thehigh-pressure vessel, it was taken out of the glove box. Thehigh-pressure vessel was connected to a gas/vacuum system, which canpump down the vessel as well as supply NH₃ to the vessel. Thehigh-pressure vessel was evacuated with a turbo molecular pump toachieve pressure less than 1×10⁻⁵ mbar. The actual pressure beforefilling ammonia was 2.6×10⁻⁶ mbar. In this way, residual oxygen andmoisture on the inner wall of the high-pressure vessel were removed.After this, the high-pressure vessel was chilled with liquid nitrogenand NH₃ was condensed in the high-pressure vessel. Approximately 40 g ofNH₃ was charged in the high-pressure vessel. After closing thehigh-pressure valve of the high-pressure vessel, it was transferred to atwo zone furnace. The high-pressure vessel was heated up. First, thefurnace for the crystallization region was maintained at 510° C. and thenutrient region was maintained at 550° C. for 24 hours. This reversetemperature setting was discovered to be beneficial for improvingcrystal quality as shown in Example 1. Then, the temperatures for thecrystallization region and the nutrient region were set to 575 and 510°C. for growth. After 4 days, ammonia was released and the high-pressurevessel was opened. The grown crystal showed yellowish color (the middlesample in FIG. 7), which represented improvement of transparency.

When 0.1 g of Ca lump was added instead of Ce, the grown crystal wassemi transparent with slight tan color (the bottom sample in FIG. 7),which represented improvement of transparency. Similar result can beexpected with adding oxygen getter containing Al, Mn, or Fe.

Example 11

In this example, a high-pressure vessel having an inner diameter of 1inch was used to demonstrate the ammonothermal growth steps with Baddition as surfactant. Unlike the Examples 1, 2, this high-pressurevessel has an open end on one side. All necessary sources and internalcomponents including 5 g of polycrystalline GaN nutrient held in a Nibasket, one 0.4 mm-thick single crystalline GaN seed, and sixflow-restricting baffles were loaded into a glove box together with thehigh-pressure vessel. The glove box is filled with nitrogen and theoxygen and moisture concentration is maintained to be less than 1 ppm.Since the mineralizers are reactive with oxygen and moisture, themineralizers are stored in the glove box at all times. 2.4 g of Na wasused as a mineralizer. After loading the mineralizer into thehigh-pressure vessel, six baffles together with the seed and nutrientwere loaded. Then, 0.1 g of BN platelet was added. After sealing thehigh-pressure vessel, it was taken out of the glove box. Thehigh-pressure vessel was connected to a gas/vacuum system, which canpump down the vessel as well as supply NH₃ to the vessel. Thehigh-pressure vessel was evacuated with a turbo molecular pump toachieve pressure less than 1×10⁻⁵ mbar. The actual pressure beforefilling ammonia was 1.8×10⁻⁶ mbar. In this way, residual oxygen andmoisture on the inner wall of the high-pressure vessel were removed.After this, the high-pressure vessel was chilled with liquid nitrogenand NH₃ was condensed in the high-pressure vessel. Approximately 40 g ofNH₃ was charged in the high-pressure vessel. After closing thehigh-pressure valve of the high-pressure vessel, it was transferred to atwo zone furnace. The high-pressure vessel was heated up. First, thefurnace for the crystallization region was maintained at 510° C. and thenutrient region was maintained at 550° C. for 24 hours. This reversetemperature setting was discovered to be beneficial for improvingcrystal quality as shown in Example 1. Then, the temperatures for thecrystallization region and the nutrient region were set to 575 and 510°C. for growth. After 4 days, ammonia was released and the high-pressurevessel was opened. The grown crystal showed less brownish color (thesecond sample from the bottom in FIG. 7), which showed improvement oftransparency. Also, the structural quality was improved compared to onewith conventional ammonothermal growth [5]. The full width half maximum(FWHM) of X-ray rocking curve from (002) and (201) planes were 295 and103 arcsec, respectively. These numbers are about 5 times smaller thancrystals reported in reference [5]. Similar result can be expected withadding surfactant containing In, Zn, Sn, or Bi.

Example 12

In this example, a high-pressure vessel having an inner diameter of 1inch was used to demonstrate the ammonothermal growth steps with Mgaddition as an oxygen getter. Unlike the Examples 1, 2, thishigh-pressure vessel has an open end on one side. All necessary sourcesand internal components including 5 g of polycrystalline GaN nutrientheld in a Ni basket, 0.4 mm-thick single crystalline GaN seeds, and sixflow-restricting baffles were loaded into a glove box together with thehigh-pressure vessel. The glove box is filled with nitrogen and theoxygen and moisture concentration is maintained to be less than 1 ppm.Since the mineralizers are reactive with oxygen and moisture, themineralizers are stored in the glove box at all times. 2.4 g of Na wasused as a mineralizer. As explained in Example 6, Na was melted in Nicrucible, capped with Ni foil and solidified. 0.1 g of Mg was added inthe Ni crucible on top of the Ni foil. After loading the Ni crucibleinto the high-pressure vessel, six baffles together with seeds andnutrient were loaded. After sealing the high-pressure vessel, it wastaken out of the glove box. The high-pressure vessel was connected to agas/vacuum system, which can pump down the vessel as well as supply NH₃to the vessel. The high-pressure vessel was evacuated with a turbomolecular pump to achieve pressure less than 1×10⁻⁵ mbar. The actualpressure before filling ammonia was 1.4×10⁻⁷ mbar. In this way, residualoxygen and moisture on the inner wall of the high-pressure vessel wereremoved. After this, the high-pressure vessel was chilled with liquidnitrogen and NH₃ was condensed in the high-pressure vessel.Approximately 40 g of NH₃ was charged in the high-pressure vessel. Afterclosing the high-pressure valve of the high-pressure vessel, it wastransferred to a two zone furnace. The high-pressure vessel was heatedup. For the first 24 hours, only the furnace for the nutrient region wasturned on and maintained at 550° C. while maintaining the furnace forthe crystallization region off in order to back etch the surface of theseed. This reverse temperature setting was discovered to be beneficialfor improving crystal quality as shown in Example 1. Then, thetemperatures for the crystallization region and the nutrient region wereset to 590 and 575° C. for growth. After 9 days, ammonia was releasedand the high-pressure vessel was opened. The grown crystal showed almosttransparent as shown in FIG. 8. Thus, reducing oxygen contamination inthe process is very effective to improve transparency of the crystal.

Example 13

Seed crystals are usually prepared by slicing bulk GaN with wire sawfollowed by polishing. There is a damage layer at the surface of theseeds created by these slicing and polishing process. To improvestructural quality of GaN, it is necessary to remove this damage layer.In addition, impurities physically or chemically adsorbed on the seedsurface can be removed with back etching. In this example, ahigh-pressure vessel having an inner diameter of 1 inch was used todemonstrate the back etching of the seed in the ammonothermalhigh-pressure vessel. Unlike the Examples 1, 2, this high-pressurevessel has an open end on one side. A GaN seed crystal was back etchedby setting the temperature of the crystallization region lower than thatof the nutrient region. All necessary sources and internal componentsincluding 10 g of polycrystalline GaN nutrient held in a Ni basket,one-0.4 mm-thick single crystalline GaN seed, and six flow-restrictingbaffles were loaded into a glove box together with the high-pressurevessel. The glove box is filled with nitrogen and the oxygen andmoisture concentration is maintained to be less than 1 ppm. Since themineralizers are reactive with oxygen and moisture, the mineralizers arestored in the glove box at all times. 4 g of NaNH₂ was used as amineralizer. After loading the mineralizer into the high-pressurevessel, six baffles together with the seed and nutrient were loaded.After sealing the high-pressure vessel, it was taken out of the glovebox. Then, the high-pressure vessel was connected to a gas/vacuumsystem, which can pump down the vessel as well as supply NH₃ to thevessel. The high-pressure vessel was heated at 125° C. and evacuatedwith a turbo molecular pump to achieve pressure less than 1×10⁻⁵ mbar.The actual pressure before filling ammonia was 1.8×10⁻⁶ mbar. In thisway, residual oxygen and moisture on the inner wall of the high-pressurevessel were removed. After this, the high-pressure vessel was chilledwith liquid nitrogen and NH₃ was condensed in the high-pressure vessel.Approximately 40 g of NH₃ was charged in the high-pressure vessel. Afterclosing the high-pressure valve of the high-pressure vessel, it wastransferred to a two zone furnace. The high-pressure vessel was heatedup. The furnace for the crystallization region was maintained off andthe nutrient region was maintained at 550° C. The actual temperaturemeasured in the crystallization region was 366° C. It is very importantto maintain the temperature difference more than 50° C. to avoid GaNdeposition on the seed due to concentration gradient. After 24 hours,ammonia was released and the high-pressure vessel was opened. The seedcrystal was back etched by 36 microns, which is sufficient to removedamage layer created by slicing and polishing.

Example 14

Although GaN has retrograde solubility at temperatures higher thanapproximately 400° C., the solubility has normal (i.e. positive)dependence on temperature below approximately 400° C. Therefore it ispossible to back etch the seed crystals by setting the crystallizationzone lower than 400° C. and maintaining the temperature ofcrystallization zone higher than that of other zones. It is important tomaintain the temperature of the nutrient region at least 50° C. lowerthan the crystallization region to avoid GaN deposition on the seed dueto concentration gradient.

Example 15

Seed crystals are usually prepared by slicing bulk GaN with wire sawfollowed by polishing. There is a damage layer at the surface of theseeds created by these slicing and polishing process. To improvestructural quality of GaN, it is necessary to remove this damage layer.In addition, impurities physically or chemically adsorbed on the seedsurface can be removed by back etching. In this example, seed crystalswere back etched in a separate reactor prior to the ammonothermalgrowth. Seed crystals with thickness of approximately 0.4 mm were loadedin a furnace reactor which can flow ammonia, hydrogen chloride, hydrogenand nitrogen. In this example, seed crystals were etched in mixture ofhydrogen chloride, hydrogen and nitrogen. The flow rate of hydrogenchloride, hydrogen and nitrogen was 25 seem, 60 sccm, and 1.44 slm,respectively. The etching rate was 7, 21, 35, and 104 microns/h for 800,985, 1000, 1050° C. Therefore, back etching above 800° C. givessufficient removal of the surface layer.

Example 16

We discovered that structural quality of grown crystals was improved byutilizing reverse temperature condition during initial temperature ramp.In this example, a high-pressure vessel having an inner diameter of 1inch was used to demonstrate the effect of reverse temperature settingin the initial stage of growth. Unlike the Examples 1, 2, thishigh-pressure vessel has an open end on one side. All necessary sourcesand internal components including 10 g of polycrystalline GaN nutrientheld in a Ni basket, 0.4 mm-thick single crystalline GaN seeds, andthree flow-restricting baffles were loaded into a glove box togetherwith the high-pressure vessel. The glove box is filled with nitrogen andthe oxygen and moisture concentration is maintained to be less than 1ppm. Since the mineralizers are reactive with oxygen and moisture, themineralizers are stored in the glove box at all times. 4 g of NaNH₂ wasused as a mineralizer. After loading the mineralizer into thehigh-pressure vessel, three baffles together with seeds and nutrientwere loaded. After sealing the high-pressure vessel, it was taken out ofthe glove box. Then, the high-pressure vessel was connected to agas/vacuum system, which can pump down the vessel as well as supply NH₃to the vessel. The high-pressure vessel was heated at 120° C. andevacuated with a turbo molecular pump to achieve pressure less than1×10⁻⁵ mbar. The actual pressure before filling ammonia was 1.5×10⁻⁶mbar. In this way, residual oxygen and moisture on the inner wall of thehigh-pressure vessel were removed. After this, the high-pressure vesselwas chilled with liquid nitrogen and NH₃ was condensed in thehigh-pressure vessel. Approximately 40 g of NH₃ was charged in thehigh-pressure vessel. After closing the high-pressure valve of thehigh-pressure vessel, it was transferred to a two zone furnace andheated up. The furnace for the crystallization region was maintained at510° C. and the nutrient region was maintained at 550° C. for 6 hours.Then, the temperatures for the crystallization region and the nutrientregion were set to 575 and 510° C. for growth. After 7 days, ammonia wasreleased and the high-pressure vessel was opened. The grown crystals hadthickness of approximately 1 mm. The structural quality was improvedcompared to one with conventional ammonothermal growth [5]. The fullwidth half maximum (FWHM) of X-ray rocking curve from (002) and (201)planes were 169 and 86 arcsec, respectively.

Example 17

We discovered that setting the crystallization temperature slightlylower than the nutrient temperature during growth significantly reducedparasitic deposition of polycrystalline GaN on the inner surface ofreactor. Although the GaN has retrograde solubility in supercriticalammonobasic solution, we discovered that GaN crystal growth occurredeven with reverse temperature condition. Moreover, structural quality ofgrown crystal was improved without sacrificing growth rate.

A high-pressure vessel having an inner diameter of 1 inch was used todemonstrate the effect of reverse temperature setting during growth.Unlike the Examples 1, 2, this high-pressure vessel has an open end onone side. All necessary sources and internal components including 10 gof polycrystalline GaN nutrient held in a Ni basket, 0.47 mm-thicksingle crystalline GaN seeds, and six flow-restricting baffles wereloaded into a glove box together with the high-pressure vessel. Theglove box is filled with nitrogen and the oxygen and moistureconcentration is maintained to be less than 1 ppm. Since themineralizers are reactive with oxygen and moisture, the mineralizers arestored in the glove box at all times. 2.6 g of Na solidified in Nicrucible and capped with Ni foil as explained in Example 6 was used as amineralizer. After loading the Ni crucible into the high-pressurevessel, six baffles together with seeds and nutrient basket were loaded.After sealing the high-pressure vessel, it was taken out of the glovebox. Then, the high-pressure vessel was connected to a gas/vacuumsystem, which can pump down the vessel as well as supply NH₃ to thevessel. The high-pressure vessel was and evacuated with a turbomolecular pump to achieve pressure less than 1×10⁻⁵ mbar. The actualpressure before filling ammonia was 9.5×10⁻⁸ mbar. In this way, residualoxygen and moisture on the inner wall of the high-pressure vessel wereremoved. After this, the high-pressure vessel was chilled with liquidnitrogen and NH₃ was condensed in the high-pressure vessel.Approximately 44 g of NH₃ was charged in the high-pressure vessel. Afterclosing the high-pressure valve of the high-pressure vessel, it wastransferred to a two zone furnace and heated up. For the first 24 hours,the furnace for the crystallization zone and nutrient zone were rampedand maintained at 360° C. and 570° C., respectively. As explained inExample 1, this reverse temperature setting ensures back etching of theseed. Then, the crystallization zone was heated and maintained to 535°C. in about 10 minutes and the nutrient region was cooled from 570° C.down to 540° C. in 24 hours. The slow cooling of the nutrient region isexpected to reduce sudden precipitation of GaN in the initial stage ofgrowth. After the temperature of the nutrient region became 540° C.,this reverse temperature setting (i.e. 535° C. for crystallizationregion and 540° C. for nutrient region) was maintained for 3 more days.Then, the ammonia was released, reactor was unsealed and cooled. Thegrown crystals had thickness of approximately 1.2 mm. The average growthrate was 0.183 mm/day. The structural quality was improved compared toone with conventional ammonothermal growth [5]. The full width halfmaximum (FWHM) of X-ray rocking curve from (002) and (201) planes were56 and 153 arcsec, respectively. Also, parasitic deposition was about0.3 g, which was more than 50% reduced compared to the conventionaltemperature setting.

Advantages and Improvements

The disclosed improvements enable mass production of high-quality GaNwafers via ammonothermal growth. By utilizing the new high-pressurevessel design, one can maximize the high-pressure vessel with sizelimitation of available Ni—Cr based precipitation hardenable superalloy.Procedures to reduce oxygen contamination in the growth system ensureshigh-purity GaN wafers. Small amount of additives such as Ce, Ca, Mg,Al, Mn, Fe, B, In, Zn, Sn, Bi helps to improve crystal quality. In-situor ex-situ back etching of seed crystals is effective way to removedamaged layer and any adsorbed impurities from GaN seeds. In addition,reverse temperature setting even for growth scheme is beneficial forreducing parasitic deposition of polycrystalline GaN on the innersurface of the reactor and improving structural quality. Thesetechniques help to improve quality of group III nitride crystals andwafers.

Possible Modifications

Although the preferred embodiment describes a two-zone heater, theheating zone can be divided into more than two in order to attainfavorable temperature profile.

Although the preferred embodiment describes the growth of GaN as anexample, other group III-nitride crystals may be used in the presentinvention. The group III-nitride materials may include at least one ofthe group III elements B, Al, Ga, and In.

Although the preferred embodiment describes the use of polycrystallineGaN nutrient, other form of source such as metallic Ga, amorphous GaN,gallium amide, gallium imide may be used in the present invention.

In the preferred embodiment specific growth apparatuses are presented.However, other constructions or designs that fulfill the conditionsdescribed herein will have the same benefit as these examples.

Although the example in the preferred embodiment explains the processstep in which NH₃ is released at elevated temperature, NH₃ can also bereleased after the high-pressure vessel is cooled as long as seizing ofscrews does not occur.

Although the preferred embodiment describes the X-ray diffraction as atesting method, the main idea can be applied to other methods such asphoto luminescence, cathode luminescence, secondary ion massspectroscopy, Hall measurement, and capacitance measurement.

Although the preferred embodiment describes the c plane wafers, thebenefit can be obtained for wafers of other orientation such as m plane,a plane and various semipolar planes.

In the preferred embodiment specific growth apparatuses is presented.However, other constructions or designs that fulfill the conditionsdescribed herein will have the same benefit as these examples.

The present invention does not have any limitations on the size of thewafer, so long as the same benefits can be obtained.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

REFERENCES

The following references are incorporated by reference herein:

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What is claimed is:
 1. A certified Group III-nitride wafer comprising(i) a documentation of a value of a physical property in combinationwith (ii) a wafer from a first set of Group III-nitride substrates cutfrom an ingot formed using a seed material, wherein the value of thephysical property is derived by a method comprising a) selectingsubstrates from the first set of substrates to form a second set ofsubstrates, the second set of substrates being a subset of the first setand therefore being a selection of substrates from the first set ofsubstrates that does not include all members from the first set ofsubstrates, the second set of substrates having at least one substrateselected from the substrates cut from a portion of the ingot located onone side of a seed used to form the ingot, and the second set ofsubstrates having at least one substrate selected from the substratescut from a second portion of the ingot located on the other side of theseed, b) analyzing samples taken from the substrates of the second setto assess values of a first property of each of the substrates of thesecond set, c) determining a correlation for the values of the firstproperty, said correlation being a function of distance of each of saidsubstrates of the second set from the seed used to form the ingot, andeither i) applying said correlation to substrates not in said second setof substrates to provide estimated values of said first property forsaid substrates; or ii) comparing said correlation against a value ofsaid property for the seed material to assess whether said value of theproperty for the seed material can be used as an estimate of theproperty for wafers cut from the ingot, and providing said value of theproperty for the seed material as the value of the property for saidwafers.
 2. A certified Group III-nitride wafer according to claim 1wherein the wafer is not a member of said second set.
 3. A certifiedGroup III-nitride wafer according to claim 1 and further includingdocumentation of a value of said physical property for the seedmaterial.
 4. A certified Group III-nitride wafer according to claim 1and further including documentation on distance from the seed material.5. A set of certified wafers comprising a plurality of wafers ofclaim
 1. 6. A certified Group III-nitride wafer according to claim 1wherein the ingot was produced using an ammonothermal process.
 7. Acertified Group III-nitride wafer according to claim 6 wherein thesubstrates of the first set each have a Group III-polar face and anitride polar face.
 8. A certified Group III-nitride wafer according toclaim 6 wherein the testing is carried out on at least one of thecrystallographic planes selected from Group III-polar side c plane,nitride-polar side c plane, m plane, or {101} plane.
 9. A certifiedGroup III-nitride wafer according to claim 6 wherein the substrate iscut at an angle to the crystallographic plane.
 10. A certified GroupIII-nitride wafer according to claim 9 wherein the angle is about 5-10degrees.
 11. A certified Group III-nitride wafer according to claim 1wherein the Group III-nitride substrates are GaN substrates.
 12. Acertified Group III-nitride wafer according to claim 1 wherein the actof analyzing samples taken from the substrates of the second setcomprises at least one analytical technique selected from the groupconsisting of X-ray analysis, X-ray diffraction analysis, electron beamanalysis, photo luminescence or cathode luminescence, secondary ion massspectroscopy, Hall measurement, capacitance-voltage measurement, andlaser light-assisted analysis.
 13. A certified Group III-nitride waferaccording to claim 1 wherein said second set includes at least oneouter-most substrate from the ingot.
 14. A certified Group III-nitridewafer according to claim 1 wherein said second set includes at least oneouter-most substrate of the original bulk crystal.
 15. A certified GroupIII-nitride wafer according to claim 14 wherein said second set includesboth outer-most substrates of the original bulk crystal.
 16. A certifiedGroup III-nitride wafer according to claim 14 wherein the outer-mostsubstrate is not a wafer.
 17. A certified Group III-nitride waferaccording to claim 15 wherein both of said outer-most substrates are notwafers.
 18. A certified Group III-nitride wafer according to claim 1wherein the act of determining the correlation additionally includes avalue of the first property for the seed material.
 19. A certified GroupIII-nitride wafer according to claim 18 wherein the correlation thatincludes the substrate from the first side of the seed differs from acorrelation that includes the substrate from the second side of theseed.
 20. A certified Group III-nitride wafer according to claim 18wherein the substrate from the first side of the seed is not a wafer.21. A certified Group III-nitride wafer according to claim 20 whereinthe substrate from the second side of the seed is not a wafer.
 22. Acertified Group III-nitride wafer according to claim 18 wherein theGroup III nitride material comprises GaN.
 23. A certified GroupIII-nitride wafer according to claim 22 wherein said GaN was formedusing an ammonothermal process.